From time to time, a user equipment (UE), such as a mobile phone or other remote terminal in a mobile radio communication system, searches for and selects cells and public land mobile networks (PLMNs). Cell and PLMN selection has a number of objectives, which include connecting a UE to the cell(s) and PLMN(s) that will provide the highest quality of service (QoS), enable the UE to consume the least power, generate the least interference, etc.
Digital communication systems include time-division multiple access (TDMA) systems, such as cellular radio telephone systems that comply with the GSM telecommunication standard and its enhancements, such as general packet radio service (GPRS) and enhanced data rates for GSM evolution (EDGE); code-division multiple access (CDMA) systems, such as cellular radio telephone systems that comply with the IS-95, cdma2000, and wideband CDMA (WCDMA) telecommunication standards; orthogonal frequency division multiple access (OFDMA) systems, such as cellular radio telephone systems that comply with the Long Term Evolution (LTE) standard; and “blended” systems. GSM, WCDMA, LTE, etc. are different radio access technologies (RATs), and the Third Generation Partnership Project (3GPP) promulgates specifications for digital communication systems that use such technologies.
PLMN and cell selection is usually based on measured received signal strength (e.g., signal to interference ratio (SIR) or signal to noise ratio (SNR)) of candidate cells. When a UE performs a PLMN Scan, the UE typically carries out three steps: a received signal strength indication (RSSI) scan, cell search on chosen frequencies, and a read of broadcast PLMN information where the cell search was successful.
For 3GPP-compliant mobile communication systems, the PLMN selection process is specified in Section 4.4 of 3GPP Technical Specification (TS) 23.122 V7.5.0, Non-Access-Stratum (NAS) functions related to Mobile Station (MS) in idle mode (Release 7) (June 2006). U.S. Patent Application Publication No. US 20070121552 by B. Lindoff for “Cell Selection in High-Speed Downlink Packet Access Communication Systems” describes a cell selection process that takes into account the delay spread of the communication channel. U.S. Patent Application Publication No. US 2002/0119774 for “Method for PLMN Selection” by R. Johannesson et al. describes how a UE receives a list of data associated with networks neighboring the PLMN currently serving the UE from a base station (BS) of the PLMN currently serving the UE. A new PLMN to serve the UE can be selected based upon the list. U.S. Patent Application Publication No. US 2008/0153486 by J. Ramkull et al. for “Efficient PLMN Search Order” describes how a UE can shorten the time needed to find a cell, such as a suitable or acceptable cell, by using intelligent search orders.
Among other things, the RSSI scan involves collecting a set of samples of received signal power or energy at several frequencies in a spectral band of interest. FIG. 1 shows an example of an RSSI scan as a plot of received power versus frequency, showing signal power peaks that might be measured by a UE on each possible carrier in the 1900 megahertz (MHz) band. An RSSI scan results in measurements within the relevant channel bandwidth (e.g., 5 MHz for WCDMA and 200 kilohertz (KHz) for GSM) on roughly 300 possible carriers in the 1900 MHz band. An RSSI scan can usually be fast. For example, it might take about 300 milliseconds (ms) for a UE to scan 300 carriers in the 1900 MHz band.
European Patent Application EP 1 367 844 A1 and U.S. Patent Application Publication No. US 2003/0236079 describe a cellular phone that includes an RSSI measurement circuit for measuring power levels of received baseband signals at divided band portions of a whole frequency band, a band sorting circuit that sorts the divided band portions based on the descending order of the power levels, and a cell search circuit that searches the carriers of each divided band portion in the order of the sorting results, to thereby determine a tentative waiting cell.
A cell search can be performed without doing an RSSI scan, but cell search is a time- and energy-consuming procedure and so it is inadvisable for a UE to perform a cell search on every possible frequency. For example, each cell search may take up to 400 ms. Cell search is traditionally based on the signal strength or SNR of candidate cells. Thus, a UE typically performs an RSSI scan first, and uses power measurements generated in the RSSI scan as an indicator of where a carrier might be found and thus where a cell search should be performed.
Nevertheless, simply using RSSI values for choosing frequencies and an order of chosen frequencies for cell search is often not efficient, especially in frequency bands that are used by several RATs. In a mixed-RAT frequency band, energy detected at a frequency may originate from a different access technology and so a cell search will fail, wasting time and energy. Another problem is that in many current implementations, all WCDMA carriers are searched before any GSM carriers are searched. Thus, when there is no WCDMA coverage in a geographic area but the spectrum is still shared between different RATs, an initial cell search will take a long time when an RSSI scan indicates relatively high power levels. Additional steps must be taken to avoid unnecessary cell search.
For example, U.S. Pat. No. 7,013,140 to Östberg et al. describes two approaches that are based on a history list and cell planning knowledge. The history list is used when a PLMN scan is triggered, and then the frequencies found in the history list are objects for cell search. For WCDMA, a frequency usually covers a large geographic area and so it is likely that a carrier found before can still can be found, but the history-list approach assumes that the WCDMA carriers are found on the same frequencies as before, which is not always true. The knowledge-of-cell-planning approach is based on a rule that a network operator typically tries to fill up a frequency band completely and on the 5-MHz width of a WCDMA carrier. Carriers (cells) are then assumed to be located with 5-MHz relative distances from the first carrier placed on the lowest frequency of the band. The likelihood of finding a PLMN/cell falls as frequencies become more distant from the assumed “maximum likelihood frequencies”.
For another example, U.S. Patent Application Publication No. US 2009/0137267 by A. Nader et al. for “Frequency Band Recognition Methods and Apparatus” discusses an algorithm for identifying WCDMA carriers. The algorithm starts off by comparing RSSI values for WCDMA carrier frequencies, or UARFCNs (UTRA Absolute Radio Frequency Channel Numbers), with a certain relative distance. The UARFCNs normally have a 200-KHz distance between each other with a few exceptions. If the RSSI values are found to have sufficiently small difference, the algorithm continues by comparing the UARFCNs that are one step closer to each other such that their RSSI values also do not differ too much, etc. Comparing two UARFCNs that have almost the same measured RSSI and continuing with two UARFCNs closer to each other and so on assumes that that the RSSI shape of the UARFCNs is more or less symmetrical, which is often true for more than just WCDMA. What is not symmetrical is probably not a WCDMA carrier, and hence a failed cell search can be avoided.
Nevertheless, assuming a UARFCN is more likely to host a WCDMA carrier than another frequency is not always valid. For example, in some frequency bands with additional UARFCNs, those additional UARFCN can be considered to be more likely. All UARFCNs are still possible, and if the frequency band is hosting more than WCDMA, the assumption might not be applicable. Thus, in bands with no WCDMA but still enough energy on every UARFCN, the UE would spend significant time and energy performing cell search.
RSSI methods can also be based on a fast Fourier transform (FFT) of the received radio signal. Using an FFT to find a spectral density is equivalent to passing the received signal through a bank of contiguous bandpass filters to detect the RSSI level. For example, each bandpass filter may be 15 KHz wide.
Most of the other prior approaches focus on identifying where to search, but do not contemplate trying to identify where NOT to search. Thus, in bands with no WCDMA carriers but still enough power on every UARFCN, the UE would spend significant time and energy performing WCDMA cell search.